The biggest problem with growing carrots is not the actual growing but the pests that attack them. Carrot root fly is the worst of these pests.1. WHAT IS A CARROT FLY?
An adult carrot fly is a very small black fly which has been described as "a low flying miniature cruise missile". The fly is attracted to the Carrots by smell. It lays it's eggs in the soil adjacent to the Carrot(s). The eggs hatch and the grubs burrow into the roots. The result is a mess, with grub tunnels all through the carrot. It stays in the ground over winter gorging itself on your Carrots, pupates and lays eggs in early spring. Eggs will ideally be laid near to Carrots but Parsley, (and Cow Parsley), Celery and Parsnips, are also affected. After the spring generation have hatched they lay eggs in June and July and this generation hatches and matures in enough time to have another frenzy of egg laying August / September time.2. HOW DO I KNOW MY CARROTS HAVE GOT THE FLY?
Basically you will not know until you lift the crop. In severe infestations the first sign is that the Carrot leaves look an orange / reddish / rusty colour. They then turn yellow. On lifting an affected Carrot it will be seen that the root end will be dark or black. Close examination of what appear to be good Carrots may reveal small holes in the Carrot. If Carrots are put in a bucket of water, badly affected ones will come to the surface. This however, does not mean, that those which do not float are totally unaffected.3. I THINK MY CROP IS AFFECTED. WHAT DO I DO?
Lift the crop. There is no point in leaving them in the ground as all you are doing is giving the carrot fly maggot an ideal home. Give the Carrots the bucket of water test and discard any which come to the surface. Of the others you may, with a bit of judicious cutting and scraping, be able to salvage something.
4. CAN I PREVENT CARROT FLY ATTACKS?
There are 5 options you may wish to try...
Apply an insecticide to the soil to prevent eggs hatching.
When sowing treat the seed drill with diazinon and chlorophos This will protect the plants for about 6 - 8 weeks. Carrots not to be lifted until the autumn should also be watered thoroughly in late August with spray strength pirimiphos-methylin.
There are few insecticides available to amateur gardeners for the following reasons. Firstly to do the job strong products are needed which are generally too toxic for amateurs to use. Secondly Carrots have a habit of accumulating pesticide in the root and therefore only chemicals which are poorly systemic are generally used. Normally the soil is treated and not the Carrot. Minor uses such as carrot fly is unlikely to be supported in the future.
At present only a few products have carrot fly as an approved use.
Chlorophos (PBI) containing diazinon & chlorophos
Soil Pest Killer (Miracle) containing pirimiphos-methyl
Pirimiphos-methyl (Sybol) drench can be used afterwards but is bound onto organic matter and will not penetrate far into the soil.
It is well known that the root fly can be put off the scent by growing strong smelling vegetables next to the carrots.
Onions or Garlic are usually used. One way to do this is to grow two rows of onions to one of carrots, the smell of the onions confuses the fly which then goes off to lay its eggs elsewhere. This only works when the onion is growing vigorously. However Carrots are an all the year crop whereas Onion and Garlic are usually of sufficient height for a couple of months of the year only. Most gardeners will have lifted their Onions and Garlic well before the fly is at it's most numerous in the early Autumn. You could try the Japanese type spring onions of the Ishikuro type that can be picked at almost any stage from small to leek size. It is also said to work the same way with the onion root fly, the smell of the carrots causes confusion and it too goes away.(For more information on companion planting)
Prevent the fly getting to the carrots
This is done by using a physical barrier.
One method is to use a frame which is about 28" tall made of stakes and polythene sheeting.
Another method is to cover the crop with horticultural fleece as soon as the first shoots appear, the carrots staying covered until picked. The use of fleece also has another benefit in that a micro-climate is produced which helps protect the plants from wind and cold. A major disadvantage for the organic grower is that unless you make a portable frame it is a real chore to continually lift the fleece to check for slugs. A frame similar to the "Geoff Hamilton Cloche" (see separate sheet) covered with fleece instead of polythene is useful as it can be lifted away for weeding etc. But make sure that you do not leave any gaps at the bottom or the flies will get through.
Use a fly resistant variety.
Some growers use varieties that will over-winter and mature before the root fly is active.Marshalls sell a variety called Sytan that is less likely to be attacked by carrot fly than other varieties. There is also a variety called 'Flyaway' from Thompson & Morgan. This is advertised as containing reduced levels of chlorogenic acid which the larvae need to survive.
You can also plant after the critical period of May-June but you cannot rely on the root fly to follow your timetable.The most popular varieties of carrots are "Early Nantes" or "Nantes Tip Top" and Nandor for the first crop sown in early March under cloches and "Autumn King" for the main crop. If using fleece or other barrier method the seeds can be sown in Feb / March and lifted in July leaving clear soil before the major problem of second generation larvae develops.
Secondly delay sowing until early June and sow thinly. The carrots are then quite young at the second generation egg laying time and are less attractive. If sown thinly they will not need to be thinned and therefore not produce the odour of damaged carrot which will really attract the fly. If the carrots are then lifted in October / November (they will be small but relatively uninfested) any remaining larvae are left without a source of food and die over winter.
It is important that if using this method, the crop is rotated each year and is not grown on the same piece of land consecutively.
ANYTHING ELSE I CAN DO?
As the fly is attracted to Carrots by smell, care should be taken when harvesting to prevent as far as possible any bruising of the foliage. Good spacing may help in this respect. As Carrot seed is notoriously difficult to sow perhaps trying pelted seeds may help.
Do not leave Carrot foliage lying around.
If you have suffered a bad attack turn over that patch of ground early in the winter. Birds and / or frost may well eat / kill any residual maggots.
Remember to rotate your crops. Do not use ground that has been freshly manured. Have a reasonable depth of clean soil with no stones otherwise you will get forked roots. Keep an eye on the plants and take action to ward off trouble.
© copyright 1999, P. A. Owen